The present invention pertains generally to targets and in particular to a source of IR radiation for targets.
Expendable sources of infrared radiation are required for purposes such as target practicing and testing. The IR radiation, in about all circumstances, must meet precise requirements as to wave length and intensity of radiation and to the spacial distribution of the radiation. If the IR source is attached to a tow target, severe restrictions on the thrust level must also be met.
There are four types of expendable IR sources and each has serious limitations. Flares have been used as expendable IR sources. They are pyrotechnic devices which burn at atmospheric pressure and produce a high-intensity, point source of radiation. These devices are generally adequate at tail-on aspect angles, but do not adequately simulate the scale factors of rocket or turbojet exhaust plumes at side-on aspect angles.
Infrared radiation has also been obtained from seeding torch flames or the exhaust of gas generators, either solid or liquid fueled. The advantages of this type are high source levels with little propulsive thrust and less of a point-source character than flares have. The disadvantages are the black-body nature of the radiation, smokiness and inadequate scale factors at side-on aspect angles. An example of this type of IR source is found in U.S. Pat. No. 3,946,555, issued on Mar. 30, 1976 to Philip J. Goede in which inorganic materials, capable of emitting after ejection a continuum infrared radiation in the wave length range of about 1 to 14 microns, are dispersed in a composite propellant.
The exhaust from conventional rocket propellants after expanding through typical converging-diverging supersonic nozzles does produce IR emissions. Unfortunately, to achieve the required plume length and IR-source strength, unacceptably high thrust levels are produced which would disturb the aerodynamics of the tow target. Also, the required mass-flow rate and desired duration may be inconsistent with the payload capability of the tow target. Encouraging afterburning to increase IR output would reduce mass-flow-rate requirements and thrust level, but afterburning in itself is an undesirable characteristic for many applications because it alters the spacial distribution of radiation in the plume, which is important for side-on aspect angles.
Another expendable infrared radiation source is a turbojet powered target drone which has carbon particles injected into its exhaust plume to augment IR output. The disadvantages are black-body spectra as opposed to line spectra of the emitted radiation and the smokiness of the exhaust. Also, this source of infrared radiation is not applicable to tow targets which are preferred over jet-powered target drones because of their reduced costs.